The production of low cost semiconductor ribbon, such as silicon ribbon, for solar cells will permit the wide scale use of such solar cells for the generation of electric power. One method of producing such materials is known as the edge-defined film-fed growth (EFG) technique and is described by Swartz et al in Journal of Electronic Materials, Vol. 4, pp. 255-279 (1975). According to this technique, a crystal is pulled upwardly from a wetted die and a capillary rise through the center of the die feeds the material to the die edge. Patents such as U.S. Pat. No. 3,591,348 to LaBelle also describe the EFG process. A reaction between molten silicon and the die is one source of degradation of the crystallographic quality of the silicon produced thereby.
Stepanov in Soviet Physics-Technical Physics, Vol. 29, pp. 339-348 (1959) describes a process wherein sheets, plates, rods and various sections of metals and alloys are produced by pulling material upwardly through a non-wetting die floating on a pool of molten metal. In the Stepanov process, because of the non-wetting of the die, there is a lack of capillary action and a certain amount of hydrodynamic drag.
Rupprecht et al, U.S. Pat. No. 3,393,054, describe an apparatus for pulling a crystal with axially oriented semiconductor crystallites from a melt. The apparatus comprises a crucible containing a melt and having a lower opening having a particular two-part nozzle made of different materials so as to provide a large temperature gradient during freezing of the melt. A heating means is in operative proximity with the crucible. By means of this pulling nozzle one is able to pull a semiconductor rod, such as a bismuth telluride rod, downwardly from the melt.
Attempts to feed the rod from above, in growing a single crystal in a downwardly direction through a non-wetting die, constituting an inverted Stepanov technique, compensate partially for the hydrodynamic drag in the slot and for the lack of capillary rise in the Stepanov technique. Attempts to use the inverted Stepanov technique for producing silicon ribbon by placing an appropriate narrow slot in the bottom of a reservoir containing a liquid silicon melt has met with indifferent success.
The present invention provides an improved apparatus for the production of such crystals.